Conventionally, a color image forming apparatus based on an electrophotographic scheme has been known, which includes a plurality of image forming units each having a laser scanner, photosensitive drum, and the like, forms images of a plurality of different colors by using the image forming units, and transfers the images over each other on a printing medium. In this type of color image forming apparatus, images formed by the image forming units must be accurately transferred over each other on the printing medium. That is, the image forming start positions of respective images on the photosensitive drum must be accurately adjusted in a main scanning direction (the direction in which the photosensitive drum is scanned by a laser) and a sub scanning direction (a direction substantially perpendicular to the main scanning direction, and corresponding to the conveying direction of the printing medium).
As a technique for adjusting the image forming start positions, the following is known. In this technique, a registration mark for misalignment correction is transferred on a feeding belt, and misalignment between the images of respective colors is detected and corrected on the basis of the read result of this registration mark. In order to correct misalignment in the main scanning direction, on the basis of the horizontal sync signal of a light beam irradiated from each laser scanner, an image write start timing for forming a latent image of each color is adjusted in each image forming unit. As a result, misalignment between the images can be corrected even when surface phases of the scanner motors are not locked with each other.
Also, in order to correct misalignment in the sub scanning direction, rotation reference signals having the same period are supplied to scanner motor control units in the respective image forming units to control rotation operation of the scanner motors. The rotational velocities of the scanner motors coincide with each other, and misalignment in the sub scanning direction can be corrected. Furthermore, the surface phases of the respective scanner motors can be kept in predetermined phases by supplying a phase difference to each rotation reference signal. By using these techniques, a sub scanning write start timing of less than one line can be corrected, thereby correcting color discrepancy.
The control sequence of the conventional scanner motor will be described next with reference to a flowchart shown in FIG. 5. First, when the scanner motor control unit receives a scanner motor activation start signal (S501), the scanner motor control unit outputs an acceleration signal to the scanner motor (S502). Upon reception of the acceleration signal, the scanner motor starts rotation (S503). Simultaneously, the scanner motor control unit counts the period of the horizontal sync signal by using a counter (S504). The scanner motor control unit then sets the rotational velocity of the scanner motor to be close to a target rotational velocity (S505) by outputting a deceleration signal if the count value is equal to or higher than the target rotational velocity, or outputting an acceleration signal if the count value is equal to or lower than the target rotational velocity.
When the rotational velocity of the scanner motor almost reaches the target rotational velocity (YES in S506), the velocity is continuously increased and decreased to lock the phases of the rotation reference signal and the horizontal sync signal (S507). When the velocity becomes almost stable, the phases of the scanner motors are locked to be in an image forming standby state (S508). Note that PLL (Phase Locked Loop) control is not performed at the time of activation of the scanner motor. This is because misalignment (registration error) due to the influence of a phase error at the image forming start position must be prevented when the phases are locked at a velocity at which a horizontal sync signal frequency is an integer multiple of a rotation reference signal frequency.
As the above-described prior art, Japanese Patent Laid-Open No. 11-218988 discloses a technique.
When a monochromatic image is to be formed, there is no possibility of color discrepancy. Hence, the scanner motor may be rotated by velocity control, and an image may be formed by irradiating the photoconductor with a laser beam when the rotational velocity reaches a predetermined target velocity. However, when a color image is to be formed, in order to prevent color discrepancy in superposing the images of the respective colors, the scanner motors must be rotated in synchronism with each other by velocity control and PLL control. Hence, when a color image is to be formed, as compared with a case wherein a monochromatic image is to be formed, a longer time is required to stabilize rotation of the scanner motor and set the image forming unit in the image forming standby state (S508). This obstructs improvement of the productivity (throughput) of the technique of forming a color image.